1. Field of the Invention
The present invention generally relates to multi-piece golf balls having a solid core of at least one layer and cover of at least one layer. A thermoplastic ionomeric composition is used to form at least one of the ball components. The composition is prepared by providing a masterbatch comprising cycloalkene (polyalkenamer) rubber and neutralizing agent and mixing the masterbatch with ethylene acid copolymer. The resulting ball has high resiliency and rebounding properties along with a soft feel.
2. Brief Review of the Related Art
Manufacturers of golf balls are constantly looking at new materials for developing multi-piece, solid balls that can be used by recreational and professional golfers. Basically, a two-piece solid golf ball includes a solid inner core protected by an outer cover. The inner core is made commonly of a rubber material such as natural and synthetic rubbers, styrene butadiene, polybutadiene, poly(cis-isoprene), poly(trans-isoprene), or highly neutralized acid copolymers. The outer cover is made commonly of ionomer resins, polyamides, polyesters, polyurethanes, or polyureas. The resiliency and rebounding performance of the golf ball is based primarily on the core of the ball. The core acts as an “engine” for the ball. In general, the rebounding performance of the ball is based on its initial velocity after being struck by the face of the golf club and its outgoing velocity after making impact with a hard surface. More particularly, the “coefficient of restitution” or “COR” of a golf ball refers to the ratio of a ball's rebound velocity to its initial incoming velocity when the ball is fired out of an air cannon into a rigid vertical plate. The COR for a golf ball is written as a decimal value between zero and one. A golf ball may have different COR values at different initial velocities. The United States Golf Association (USGA) sets limits on the initial velocity of the ball so one objective of golf ball manufacturers is to maximize COR under these conditions. Balls with a higher rebound velocity have a higher COR value. Such golf balls rebound faster, retain more total energy when struck with a club, and have longer flight distance.
Other multi-piece golf balls are known in the industry. For example, a four-piece solid golf ball having an inner core and surrounding outer core layer (dual-core) with an intermediate layer and outer cover may be made. Five-piece balls having a dual-core, surrounding intermediate layer, and multi-layer cover comprising an inner cover and outer cover also are known. Different materials can be used to impart specific properties and features to the balls.
The patent literature describes various golf balls having multi-layer core constructions. For example, Sullivan et al., U.S. Pat. No. 7,722,482 discloses a golf ball having a multi-layer core and cover. The multi-layer core consists of a center and an outer core layer that are both soft relative to a hard intermediate core layer. The outer core layer is thin relative to the center and the outer core layer. Sullivan et al., U.S. Pat. No. 6,852,044 discloses golf balls having multi-layered cores including a relatively soft, low compression inner core surrounded by a relatively rigid outer core. Ohsumi et al., U.S. Pat. No. 5,772,531 discloses a solid golf ball including a solid core having a three-layered structure composed of an inner layer, an intermediate layer, and an outer layer. The intermediate layer is designed to have a JIS-C hardness of 50 to 80, and the outer layer is designed to have a hardness which is higher than the hardness of the intermediate layer.
It is also know that cycloalkene (polyalkenamer) rubber compositions can be used to form the core. For example, Kim et al., U.S. Pat. No. 7,528,196 and U.S. Patent Application Publication US 2009/0191981 disclose a golf ball comprising a core, cover layer, and optionally one or more inner cover layers, wherein at least one portion of the ball comprises a blend of a polyalkenamer and polyamide. The polyalkenamer/polyamide composition contains about 2 to about 90 weight % of a polyalkenamer polymer and about 10 to about 98 weight % of a polyamide. The '196 patent and '981 Published Application further disclose that the polyalkenamer/polyamide composition may be blended with other polymers including rubbers such as polybutadiene, polyisoprene, polychloroprene, polybutylene, and styrene-butadiene rubber and ionomers.
Ionomeric materials have long been used as layers of golf balls, particularly as inner or outer cover layers. In general, ionomers refer to copolymers of α-olefin; C3 to C8 α, β-ethylenically unsaturated mono- or dicarboxylic acid; and optional softening monomer. Typically, the α-olefin is ethylene or C3 to C8 and the vinyl comonomer has acid group such as methacrylic, acrylic acid, or maleic acid. These ionomers may be prepared by methods known in the art. These ionomer acid copolymers contain inter-chain ionic bonding. Metal ions such as sodium, lithium, zinc, and magnesium are used to neutralize the acid groups in the copolymer. Commercially available olefin-based ionomer resins are available in various grades and identified based on the type of base resin, molecular weight, and type of metal ion, amount of acid, degree of neutralization, additives, and other properties. Ionomers have excellent toughness, crack resistance, resilience, and hardness which make them ideally suited for these types of layers.
Most ionomers suitable for conventional golf ball layers have a percent neutralization of from 19 wt % to 69 wt %. Neutralization helps cause cross-linking to occur and this ionic cross-linking helps impart desirable properties to the material such as durability and hardness. However, as the level of neutralization increases, the viscosity of the ionomer resin increases and this makes it difficult to mold the material into covers for the ball. Thus, although the properties of the highly neutralized ionomer resins may be advantageous, the melt processability of such resins is generally poor. Neutralization to 90% and higher is known but at these levels, the material has a very low melt flow and may even be intractable. As a result, high levels of metal cation-fatty acid flow modifiers often need to be added to such resins. For example, Statz et al., U.S. Pat. No. 6,777,472 generally describes a process for modification of highly-neutralized ionomers by the addition of a sufficient amount of specific organic fatty acids (or metal salts thereof) in order to maintain melt-processability. Rajagopalan et al., U.S. Pat. No. 6,756,436 discloses a golf ball containing a core made of a polymer containing an acid group neutralized by an organic acid or a salt thereof, the organic acid or salt thereof being present in an amount sufficient to neutralize the polymer by at least about 80%.
Bulpett et al, U.S. Patent Application Publication US 2009/0227394 discloses that ionomer resins may be a useful material for making a thermoplastic intermediate core layer in a multi-layer core having a thermoset inner core layer (center) and thermoset rubber outer core layer. When ionomeric resins are modified to relatively high levels (greater than 75%), the ionomers show reduced melt flow and improved heat resistance making them useful material for such intermediate core layers. However, there may be problems with adding metal oxides or other neutralizing agents to ethylene acid copolymers to provide high levels of neutralization. For example, the process normally involves directly adding caustic powders and/or liquids to the ethylene acid copolymer via a twin screw extruder which can be difficult, messy, and dangerous.
In another method, Neill et al., U.S. Pat. Nos. 4,666,988 and 4,774,290 disclose a process for forming melt-reacted blends of ethylene/carboxylic acid copolymers with metal oxides are improved by first incorporating the metal oxide into an ethylene acid copolymer of low density polyethylene (LDPE), high density polyethylene (HDPE), or linear low density polyethylene (LLDPE) to form a concentrate or masterbatch, then melt-blending a minor amount of the concentrate into the ethylene acid copolymer.
It would be desirable to develop a new method for introducing neutralizing agents into molten ethylene acid copolymers that can be done neatly, safely, and with greater control. Another objective of the present invention is to develop compositions that can be used to make cores and covers for a golf ball, wherein the core provides the ball with high resiliency along with a comfortable and soft feel. Yet another goal involves preparing a thermoplastic ionomeric composition having good durability and strength, that could be used in packaging, surface coatings, and other applications. The present invention provides such methods, components for golf balls, and compositions having such properties as well as other advantageous characteristics, features, and benefits.